Conventional systems for applying electrically charged non-magnetic toner to substrates, such as shown in U.S. Pat. Nos. 5,633,108, 5,656,409, and 5,532,100 (the disclosures of which are hereby incorporated by reference herein) provide a number of advantages compared to conventional constructions and methods for applying toner to substrates. Typically, a single toner bed is maintained in a fluidized state, with a toner level sensor using a feedback circuit controlling a replenishment system. While the present system and method are highly advantageous, there are some problems associated therewith which desirably should be overcome, and are overcome utilizing the system and method of the present invention.
One of the problems of the conventional system is the level control. The sensor and feedback group construction can guarantee an accuracy of only .+-.0.02 inches. A deviation this large causes a cyclic change in output print density. Also, the present system experiences problems with level control and start-up and emergency stoppages, which can cause downtime and operator frustration.
The second problem with the present system is the existence of wrong sign toner. That is negatively charged toner particles can collect in the sole toner bed, disrupting the flow in the bed and causing electrical field charges in the unit. Then the print quality can drift because of this, and more frequent maintenance intervals are needed.
The third problem with conventional systems is dusting. Because of conventional delivery roller system geometry, a high amount of vacuum cleaning is needed. This disrupts the air management within the unit and can cause level fluctuations, and unexpected toner vectoring at the ends of the rollers.
The fourth problem with conventional systems is an uneven toner layer. In the present system and method it is difficult to properly control the consistency and thickness of the toner layer so as to insure uniform application of toner to the substrate.
According to the present invention a system and method are provided which overcome the above problems. The invention utilizes a dual bed system, with a supply bed and a feeder bed at different levels (additional beds may also be used). Three (or more) system rollers are utilized, in addition to the image cylinder, to transport charged toner from one bed to the next, and upwardly away from the bed to the image cylinder. Toner polarity filtration is maximized as the toner is lifted and transferred into the feeder bed. Essentially absolute toner level control (an accuracy of better than 0.01 inches) is provided in the feeder bed using a dam spillway system, and improved toner scatter control is provided using an over/under configuration of toner delivery rollers.
According to a first aspect of the present invention a system for applying electrically charged non-magnetic toner to an image device (and then subsequently to a substrate) is provided. The system comprises or consists essentially of the following components: A first fluidized toner bed having electrically charged non-magnetic toner maintained at substantially a first level therein. A first corona source disposed in the first bed for electrically charging toner in the first bed. A second fluidized toner bed adjacent the first bed and having toner maintained at substantially a second level therein, different than the first level. A second corona source disposed in the second bed for electrically charging toner in the second bed. An image device. A first toner transfer mechanism maintained at a first electrical bias for transferring toner from the first bed to the second bed. And a second toner transfer mechanism maintained at a second electrical bias different than the first bias for transferring toner from the second bed to the image device, the image device maintained at a third electrical bias different than the first and second biases.
The first toner transfer mechanism may comprise a first conductive roller rotatable in a first direction, although other transfer mechanisms may be utilized, such as a pneumatic toner lifting chute. A second toner transfer mechanism preferably comprises a second conductive roller rotatable in a first direction and operatively engaging the first conductive roller. Also, the second toner transfer mechanism typically further comprises an opposing roller above the axis of rotation of the second roller and operatively engaging the second conductive roller and rotatable in a second direction opposite the first direction, the opposing roller operatively engaging the image device. Typically the image device comprises an image roller rotatable in the first direction.
Preferably the second level of toner is maintained in the second bed by a spillway-defining restraining dam disposed between the first and second beds so that the toner above the second level spills over the restraining dam into the first bed, although any conventional structure for that purpose (and preferably with an accuracy of 0.01 inches is the maximum level) may be utilized. A single toner replenishment device is preferably provided for replenishing the first bed with toner when the level of toner therein falls below the first level. While a single replenishment device is preferably provided, it may have multiple discharges into the first toner bed if desired.
The system may further comprise a first scraper for scraping excess toner from said first conductive roller so that the scraped toner falls back into said first bed; a second scraper for scraping excess toner from the second conductive roller so that the scraped toner falls back into the second bed; and a third scraper for scraping excess toner from the opposing roller so that the scraped toner falls back into the second bed. The first and second corona devices may comprise corona wires, spiked rotating wheels, or any other conventional constructions. In one preferred embodiment, the first electrical bias is between about 600-1200+ volts (e.g. about 800+ volts), the second electrical bias is between about 100-500+ volts (e.g. about 400+ volts), the opposing rollers are substantially at above ground potential, and the third electrical bias is a negative bias, so the toner transfers electrostatically from the first conductive roller to the second conductive roller to the opposing roller and to the image device.
According to another aspect of the present invention a method of applying conductive non-magnetic toner to a moving substrate using first, second, opposing, and image conductive rotating cylinders (rollers) with the axis of rotation of the opposing cylinder above those of the first ands second cylinders, and the axis of rotation of the image cylinder above that of the opposing cylinder, each cylinder having a peripheral surface, is provided. The method comprises: (a) Biasing the first conductive cylinder to a first electrical bias, and rotating it in a first direction so that conductive non-magnetic toner is attracted to the peripheral surface thereof. (b) Biasing the second conductive cylinder to a second electrical bias different than the first bias and rotating it in the first direction, so that conductive non-magnetic toner is transferred from the first cylinder to the second cylinder. (c) Biasing the image cylinder to a third electrical bias different than the first and second biases, and rotating it in the first direction so that conductive non-magnetic toner is transferred from the opposing cylinder to the image cylinder. (d) Biasing the opposing cylinder to a fourth electrical bias different than the first, second, and third biases, and rotating it in a second direction opposite the first direction, so that conductive non-magnetic toner is transferred from the second cylinder to the opposing cylinder. And (e) transferring the toner from the image cylinder to a moving substrate to form images on the substrate.
Preferably, (a)-(d) are practiced so that the first electrical bias is about 600-1200+ volts, the second electrical bias is about 100-500+ volts, the fourth electrical bias is substantially at about ground potential, and the third electrical bias is a negative bias. The method may further be practiced utilizing first and second beds of toner, and then preferably (a) is practiced by transferring toner from the first bed to the first cylinder, and (b) is practiced by scraping toner from the second cylinder so that it is deposited in the second bed, and then transferring toner from the second bed to the second cylinder; and further comprising (f) scraping excess toner from the opposing cylinder so that it falls into the second bed. The method may further comprise (g) electrically charging the conductive non-magnetic toner in the first and second beds using corona devices; (h) maintaining the maximum level of toner in the second bed with an accuracy of 0.01 inches or better; and (i) replenishing the toner in the first bed when it falls below a predetermined level. Preferably, (h) is practiced by providing a spillway defined by a restraining dam between the first and second beds so that any excess toner in the second bed spills over into the first bed.
The invention further comprises a method of applying electrically conductive nonmagnetic toner to a moving substrate using first and second toner beds, and an image device comprising or consisting essentially of: (a) Replenishing the electrically conductive non-magnetic toner in the first bed when it falls below a predetermined level. (b) Transferring toner from the first bed to the second bed. (c) Maintaining the maximum level of toner in the second bed with an accuracy of 0.01 inches or better. (d) Transferring toner from the second bed to the image device to provide a substantially even toner layer on the image device. And (e) transferring toner from the image device to a moving substrate. Typically, (c) is practiced by providing a spillway defined by a restraining dam between the first and second beds so that any excess toner spills over from the second bed into the first bed. Also, the method further comprises maintaining the first and second toner beds as fluidized beds, and electrically charging the toner in the first and second fluidized beds.
It is the primary object of the present invention to provide an advantageous system and method for applying electrically charged non-magnetic toner to an image device, and subsequently to a substrate. This and other objects of the invention will become clear from an inspection from the detailed description of the invention and from the appended claims.